Conventionally, a TTL autofocus (to be referred to as AF hereinafter) system has been proposed, in which a high frequency component is extracted from a captured signal, a search is then made for a place where it is maximized (the top of a hill), and the place is determined as an in-focus point. AF based on this system will be referred to as captured signal AF hereinafter.
According to the characteristic features of this system, it requires no mechanical members or the like for automatic focus detection, and hence can be implemented at a low cost. In addition, since focus determination is performed by using a captured signal itself, the system is accurate, free from deterioration with time, and the like. Captured signal AF which is free from the influence of deterioration of mechanical members with time is very effective for image capturing operation by a high-pixel count image capturing device because it requires severe focusing accuracy.
In this system, in order to discriminate the top of a hill, i.e., an in-focus point, the entire region is scanned from end to end to comprehend its shape or so-called hill-climbing operation is performed. Comprehending the shape of a hill by scanning makes the operator see in-focus and out-of-focus events. This operation is therefore inappropriate for a state wherein moving images are captured or previewed on a monitor. Hill-climbing operation is often used for moving image capturing or monitor previewing operation. When, however, this operation starts from a lens position distant from an in-focus point, it starts from a flat portion of a hill. It is therefore difficult to discriminate the direction of the top of the hill. If a mistake is made on the direction of the top of the hill, awkward operation occurs, for example, a search goes to an end opposite to the top of the hill and then returns. In addition, it takes much time to achieve focusing.
Since autofocus based on TTL or a direct distance measurement system (to be referred to as direct distance measurement AF hereinafter), which uses an infrared triangular distance measurement system or pupil division phase difference detection system, can directly measure a distance, scanning operation or hill-climbing operation is not required to detect the position of a in-focus point. This technique allows fast discrimination of an in-focus point. However, the technique requires mechanical members and the like for automatic focus detection, and distance measurement is performed by using a system different from the image capturing system, and hence has the drawbacks of being vulnerable to a shift in in-focus position due to deterioration with time or temperature, causing parallax in a captured image and distance measurement position in accordance with the distance, and the like. When the technique is applied to a high-pixel count image capturing device, in particular, a shift in in-focus position due to deterioration with time or temperature may sometimes lead to critical blurring.
As a means which improves AF performance, hybrid AF has been proposed, which is obtained by combining captured image AF and direct distance measurement AF to realize fast, accurate autofocus by combining the merits of the two AF systems. As a selection method for these two AF systems, a system of switching from external distance measurement AF to captured signal AF in response to the operation of an image capturing switch has been proposed (see, e.g., Japanese Patent Laid-Open No. 2001-264622).
This system is suitable for still image capturing operation but is not suitable for moving image capturing operation of sequentially changing objects to be image-captured in accordance with panning and the like without operation of the image capturing switch. In addition, there has been proposed a method in which in-focus positions are detected by using the two systems, and when the measurement results obtained by the two systems differ by a predetermined amount upon comparison, one of the systems is selected in accordance with a condition (see Japanese Patent Laid-Open No. 2001-264622). This system is, however, based on the system of performing scanning in advance to detect an in-focus position, and is not suitable for AF for moving images. This is because scanning operation is noticeable, and undesirable images are obtained.
There has also been proposed a technique of driving a lens toward the front-focused/rear-focused position by a predetermined amount by direct measurement AF, then switching to captured signal AF, and if contrast detection cannot be done after switching, switching again to direct measurement AF (see Japanese Patent Laid-Open No. 2001-141984).
There has further been proposed a technique of performing direction discrimination/motor speed control by the phase difference system if the focus has greatly shifted, and performing AF by captured image AF if a high frequency component is detected (see Japanese Patent Laid-Open No. 3-81713).
Since the TV systems are limited to NTSC, PAL, and the like, conventional digital video cameras designed to capture moving images generally perform interlaced image capturing (60I) at a rate of almost 60 fields per sec according to the NTSC system, and perform interlaced image capturing (50I) at a rate of almost 50 fields per sec according to the PAL system. Image capturing devices are also driven in synchronism with these periods. However, with generalization of TV digital broadcasting, high-definition broadcasting, movie digital image capturing, and the like, digital video cameras have been designed to perform image capturing in various image capturing formats.
With regard to TV broadcasting systems alone, with resolutions of 480i, 480p, 720p, and 1080i, some area is assigned 60 fields/frames, and the other area is assigned 50 fields/frames. Note that in the case of progressive scanning, 60 frames and 30 frames are respectively expressed as 60P and 30P using frame counts. In the case of interlaced scanning, 60 fields and 50 fields are respectively expressed as 60I and 50I using field counts. Consider a case wherein moving images are replayed on a personal computer. Moving images with frame counts less than 30 (e.g., 20P, 15P, and 10P) are generally captured by progressive image capturing. In addition, in consideration of compatibility with film movies, image capturing at a rate of 24 frames per sec, i.e., 24P image capturing, is becoming popular.
High-sensitivity image capturing is performed as image capturing in dark places and the like. In this case, a moving image capturing technique called slow-shutter image capturing is generally performed, in which long-time exposure image capturing is performed by decreasing the field/frame count of an image capturing device to a count smaller than the recording field/frame count. In this case, although the field/frame count of the image capturing device is decreased for long-time exposure, the field/frame count of a final video signal is kept constant in conformity with a broadcasting system or TV interface.
When conventional captured signal AF is used for moving image capturing, a moving captured signal used for display and recording is generated, and at the same time AF evaluation value is obtained from part of the signal. The amount of obtained information depends on the field count/frame count of a captured image. When the field count/frame count of a captured image is large, smooth focusing operation can be performed. If, however, the field count/frame count is small, the focusing speed decreases, resulting in difficulty in performing smooth focusing operation. In addition, the follow-up speed decreases with respect to a moving object. In the case of captured signal AF for a still image, the frame count can be temporarily increased by changing the driving method for the image capturing device for AF only at the time of AF. However, such a technique cannot be applied to moving image capturing because AF and moving image recording are simultaneously performed.